In order to allow a semiconductor device to be high in breakdown voltage and low in loss and to be used in a high-temperature environment, silicon carbide has recently increasingly been adopted as a material forming a semiconductor device. For example, Mitsuo Okamoto et al., “Reduction of Instability in Vth of 4H—SiC Carbon Face MOSFETs,” the 59th Spring Meeting, Proceedings, The Japan Society of Applied Physics, Spring 2012, pp. 15-309 (NPD 1) has pointed out that a silicon carbide metal oxide semiconductor field effect transistor (MOSFET) suffers from fluctuation in threshold voltage due to gate bias stress. This document discloses a method of annealing a silicon carbide substrate having a gate oxide film formed thereon in a hydrogen atmosphere in order to reduce fluctuation in threshold voltage.